The following publications are representative of the most relevant prior art known to Applicants at the time of filing of this application:
______________________________________ United States Patents ______________________________________ 4,572,439 Feb. 25, 1986 E. W. Pitzer 4,756,746 July 12, 1988 P. B. Kemp et al. ______________________________________ Foreign Patents ______________________________________ JP 60/46963 March 14, 1985 Harima Refractories ______________________________________
Alumina particles in general are well known. They have been used in many applications including as fluid bed catalyst support materials for a variety of chemical synthesis procedures, e.g. hydrogenation, dehydrogenation, dehydration, dehydrocyclization and the like. For example, the Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, Volume 10, pp 550-553, lists over 100 processes employing fluidized beds. Alumina particles may be produced having a wide variation in properties such as in alumina content, surface area, attrition resistance, particle size distribution, density, shape and porosity. Generally, the usefulness of particular alumina particles depends upon the type and severity of environment that they can withstand in use without degradation.
Particles or granules, including those of alumina, have been commercially prepared in generally spherical shapes by various processes including pelleting in liquid media during gravity flow down, gelling from alumina-containing dispersions, and plasma spraying; c.f. U.S. Pat. No. 4,756,746, Japanese Kokai No. 60/46,963. The difficulty is not in producing spherical alumina particles, but rather in producing essentially spherical alumina particles having the specific combination of properties which will enable the particles to be especially useful in steam-laden atmospheres at elevated temperatures.
Norton Company has sold commercially several low surface area fluidizable catalyst carriers, specifically types 06595 and 06596. These carriers, however, have air jet attrition rates of about 0.6-0.8 weight %/hr. As shown below, the carriers of the present invention offer substantially increased attrition resistance without increasing the silica content and also without adversely altering the bulk density and porosity. Increased attrition resistance is extremely important commercially since it reduces catalyst make up rates and therefore lowers unit operating costs.
The production of alumina particles having high attrition resistance can be relatively easily accomplished by merely separating the attrition resistant particles from a mixture of the same with frangible particles by using acoustic energy to disintegrate the frangible ones, c.f. U.S. Pat. No. 4,572,439. Alternatively and more generally, increased attrition resistance is accomplished by producing hardened particles which are more dense. The particles of the present invention, however, have a relatively low tap density of less than about 1.9 g/cc while simultaneously having an attrition resistance of less than about 0.30 weight percent/hour. It is also possible to manufacture more attrition resistant alumina particles by merely increasing their silica contents. This, however, makes the particles unsuitable for use in steam-laden environments due to the silica being leached from the particles and causing fouling of downstream equipment.
It is thus an object of the present invention to produce particles consisting essentially of alumina, which particles have a very low surface area, a high attrition resistance, a narrow particle size distribution, a relatively low density, a generally spherical shape, and also sufficient porosity to hold catalytic metals.
It is a further object to produce an essentially pure alumina particle which will have sufficient integrity to be used in fluid bed reactors operated under constant or varying exposure to steam such as in steam reforming or regeneration of hydrocarbon conversion catalysts and other such processes. It is particularly important for such operations to minimize the silica content of the particles due to the silica leaching problem.